Fix #145: tree membership management (list / add / role / remove)

TreeMembership was enforced on every read/write but had no API or UI to manage
members — trees were effectively single-user, breaking full-CRUD (NN#8).

Backend (/trees/{id}/members): list (members only — the list exposes emails, so
non-members never see it, even on public trees); add an existing user by email
(owner only, 404 if no such account, 409 if already a member); PATCH role;
DELETE. A tree must always keep ≥1 owner (demote/remove of the sole owner → 409).
All changes audited.

Frontend: a Members page (owner gets add-by-email + per-member role select +
remove; others see a read-only list) and a sidebar entry.

Test covers the full lifecycle + every guard. Suite 77 passed.

Closes #145

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/schemas/membership.py

@@ -0,0 +1,26 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+from app.models.enums import MembershipRole
+
+
+class MembershipRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    user_id: uuid.UUID
+    email: str
+    display_name: str | None
+    role: MembershipRole
+    created_at: datetime
+
+
+class MemberAdd(BaseModel):
+    email: str
+    role: MembershipRole = MembershipRole.viewer
+
+
+class MemberRoleUpdate(BaseModel):
+    role: MembershipRole